TSRs have evolved over the last 70 years, with the greatest degree of its evolution occurring within the past 20 years. The understanding of the complexity of the shoulder has resulted in the ability to better treat the multiple conditions that afflict the shoulder. Glenohumeral arthritis ranges from simple to complex due to etiology and deformity. Post traumatic glenohumeral arthritis, along with the deformity of both the glenoid and humeral head present challenges for the shoulder arthroplasty surgeon. Similarly, the problem of rotator cuff deficiency and rotator cuff arthropathy has resulted in the development of treatment and prosthetic design specific to address the loss of the main motors of the shoulder.
Currently, there are two types of TSR—traditional anatomic total shoulder replacement (ta-TSR) and reverse total shoulder replacement (r-TSR). Ta-TSR utilizes resurfacing of the humeral head and glenoid in the setting of an intact and functioning rotator cuff. Glenohumeral arthritis has been treated with ta-TSR, the current gold standard being the resurfacing of the humeral head with a stemmed or metaphyseal component along with a replacement of the humeral head articular portion with a Cobalt-Chromium (Co—Cr) implant. Modularity of the humeral components allows for appropriate sizing of the head in diameter and thickness to match the resected articular surface of the patient.
To revise from the ta-TSR to r-TSR often requires removal of the glenoid component and reconstruction of the glenoid bone stock. Ta-TSR utilize all polyethylene glenoid components which become the typical point of failure for the ta-TSR. R-TSR utilizes a porous in-growth metal design with locking screws for glenoid fixation. Modularity has always centered on being able to change the humeral components from ta-TSR to r-TSR, for example, a humeral head component to a socket configuration.
The resurfacing of the glenoid has evolved over the past 70 years. Originally, polyethylene bonded to metal, also known as metal-backed glenoids, was cemented into the glenoid bone. These failed at the polyethylene-metal interface due to stresses and edge loading of the component. What evolved was the use of all polyethylene components. First, all polyethylene with a keel was used, followed by all polyethylene with multiple pegs.
There was a higher rate of failure for the cemented keeled components, so currently the gold standard for glenoid resurfacing in ta-TSR is a cemented pegged, all polyethylene component.
The most common cause of failure of the ta-TSR is due to glenoid loosening secondary to rotator cuff failure/tear. The resulting superior migration of the humeral head, with concomitant change in the center of rotation (C.O.R.) from rotator cuff failure produces edge loading of the glenoid component. This asymmetric mechanical loading results in rocking and loosening of the polyethylene prosthesis from the cement and bone of the glenoid.
R-TSR evolved from the specific abnormal mechanics of the rotator cuff deficient shoulder, as previously described. In the rotator deficient condition, the deltoid muscle becomes the predominant motor, but in an inefficient manner. The deltoid muscle contraction functions to result in “hinged abduction” of the humeral head/humeral shaft. The humeral head and greater tuberosity lever on the undersurface of the acromion and superior portion of the glenoid. Ta-TSR is contra-indicated in the setting of rotator cuff deficiency, due to the known catastrophic results to the glenoid component.
The development of the r-TSR addresses the rotator cuff deficient, painful arthritic shoulder. The design of r-TSR is to maximize deltoid fiber length to allow more efficient contraction and function of the deltoid in elevation of the arm. The prosthetic components are designed to change the C.O.R. to one that is more inferior and medial to the native joint.
The design of r-TSR has also evolved over the past 20 years. The original “Grammont” style sought to inferiorly displace the humerus to maximize deltoid fiber length; this resulted in inferior scapular notching, leading to failure. The current revised designs include a C.O.R. which is more lateral and inferior to the native C.O.R. The implant design is for an in-growth trabecular metal baseplate with locking screws to secure the component to the bony glenoid. The relatively minimally curved glenoid is replaced with a glenosphere: a solid Co—Cr semi-spherical to spherical surface that attaches to the in-growth metal base-plate. This is typically through a combination of a Morse taper fit and center screw fixation. The glenosphere is typically inserted at an inferiorly directed version angle, between 5-10 degrees. This allows for inferior offset of the humerus, elongation of the deltoid muscle fibers and a joint reactive force in line with prosthetic alignment.
R-TSR already have asymmetric, higher shear and higher loading of the glenoid component, called the glenosphere and baseplate construct. Despite these greater loads, in growth metal baseplates with locking screws are not the cause of failure, due to the excellent bone incorporation and stability.
The r-TSR has a different humeral component design as well. Where the ta-TSR has the Co—Cr humeral head, the r-TSR had the Co—Cr glenosphere attached to the glenoid. The humerus had a stemmed component but attached to the top is a polyethylene cup or humeral cup to articulate with the glenosphere. The modularity of components, specifically glenosphere sizing and humeral cup sizing, allow for multiple permutations to achieve the most successful and stable construct.
Current long-term studies on viability of r-TSR have revealed that the construct of an in-growth metal baseplate with locking screws has excellent long-term fixation without evidence of loosening, even in osteoporotic bone.
Currently, there is not a single company known to the inventor that has a universal baseplate for both ta-TSR and r-TSR. Further, the modularity that exists today allows only for conversion of a ta-TSR humeral stem to accommodate a r-TSR humeral cup. Conversion to a r-TSR requires removal of the polyethylene glenoid with reconstruction of the glenoid bone with tricortical bone, often taken from the patient's own iliac crest, and consequential donor site morbidity.